METHOD AND SYSTEM TO DETERMINE PRESSURE IN AN UNFIRED CYLINDER

Abstract

An article of manufacture and method are provided to determine pressure in an unfired cylinder of an internal combustion engine. The cylinder comprises a variable volume combustion chamber defined by a piston reciprocating within a cylinder between top-dead center and bottom-dead center points and an intake valve and an exhaust valve controlled during repetitive, sequential exhaust, intake, compression and expansion strokes of said piston. The code is executed to determine volume of the combustion chamber, and determine positions of the intake and exhaust valves. A parametric value for cylinder pressure is determined at each valve transition. Cylinder pressure is estimated based upon the combustion chamber volume, positions of the intake and exhaust valves, and the cylinder pressure at the most recently occurring valve transition.

Full Text

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METHOD AND APPARATUS TO DETERMINE PRESSURE IN AN UNFIRED CYLINDER
TECHNICAL FIELD
[0001] This invention pertains generally to control systems for engine and
powertrain systems.
BACKGROUND OF THE INVENTION
[0002] Internal combustion engines are employed on various devices,
including mobile platforms, to generate torque for traction and other
applications. An internal combustion engine can be an clement of a
powertrain architecture operative to transmit torque through a transmission
device to a vehicle driveline. The powertrain architecture can further include
one or more electrical machines working in concert with the engine. During
ongoing operation of the mobile platform employing the internal combustion
engine, it may be advantageous to discontinue firing one or more of the
cylinders, including stopping engine operation and engine rotation completely.
It may be further advantageous to subsequently have knowledge of pressure
within the cylinder, to effectively spin, fire, and restart the engine during
ongoing operation, to control and manage engine torque vibration, reduce
noise, and improve overall operational control of the powertrain.
[0003] Prior art systems use models developed off-line to determine cylinder
pressure. Such systems are advantageous in that they minimize need for real-
time computations. However, such systems have relatively poor accuracy, due
to variations introduced by real-time variations in factors including
atmospheric pressure, engine speed, initial engine crank angle, engine wear
characteristics, and others. Therefore, there is a need to accurately determine
engine cylinder pressure in real-time during ongoing operation of the engine.

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SUMMARY OF THE INVENTION
[0004] In accordance with an embodiment of the invention, an article of
manufacture and method are provided, comprising a storage medium having
machine-executable code stored therein. The stored code is to determine
pressure in an unfired cylinder of an internal combustion engine. The cylinder
comprises a variable volume combustion chamber defined by a piston
reciprocating within a cylinder between top-dead center and bottom-dead
center points and an intake valve and an exhaust valve controlled during
repetitive, sequential exhaust, intake, compression and expansion strokes of
said piston. The code is executed to determine volume of the combustion
chamber, and determine positions of the intake and exhaust valves. A
parametric value for cylinder pressure is determined at each valve transition.
Cylinder pressure is estimated based upon the combustion chamber volume.
positions of the intake and exhaust valves, and the cylinder pressure at the
most recently occurring valve transition.
[0005] These and other aspects of the invention will become apparent to
those skilled in the art upon reading and understanding the following detailed
description of the embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] The invention may take physical form in certain parts and
arrangement of parts, an embodiment of which is described in detail and
illustrated in the accompanying drawings which form a part hereof, and
wherein:
[0007] Fig. 1 is a schematic diagram of an exemplary engine, in accordance
with the present invention; and,
[0008] Fig. 2 is a schematic diagram of an exemplary control scheme, in
accordance with the present invention.

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DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION
[0009] Referring now to the drawings, wherein the depictions arc for the
purpose of illustrating the invention only and not for the purpose of limiting
the same, Fig. 1 depicts a schematic of an internal combustion engine 10 and
control system 5 which has been constructed in accordance with an
embodiment of the present invention. The engine is meant to be illustrative,
and comprises a conventional fuel-injection spark ignition engine. It is
understood that the present invention is applicable to a multiplicity of internal
combustion engine configurations.
[0010] The exemplary engine comprises an engine block 25 having a
plurality of cylinders and a cylinder head 27 is sealably attached thereto.
There is a moveable piston 11 in each of the cylinders, which defines a
variable volume combustion chamber 20 with walls of the cylinder, the head,
and the piston. A rotatable crankshaft 35 is connected by a connecting rod to
each piston 11, which reciprocates in the cylinder during ongoing operation.
The cylinder head 27 provides a structure for intake port 17. exhaust port 19.
intake valve(s) 21, exhaust valve(s) 23, and spark plug 14. A fuel injector 12
is preferably located in or near the intake port, is fluidly connected to a
pressurized fuel supply system to receive fuel, and is operative to inject or
spray pressurized fuel near the intake port for ingestion into the combustion
chamber periodically during ongoing operation of the engine. Actuation of the
fuel injector 12, and other actuators described herein, is controlled by an
electronic engine control module ('ECM'). which is an element of the control
system 5. Spark plug 14 comprises a known device operative to ignite a
fuel/air mixture formed in the combustion chamber 20. An ignition module,
controlled by the ECM, controls ignition by discharging requisite amount ol~
electrical energy across a spark plug gap at appropriate times relative to
combustion cycles. The intake port 17 channels air and fuel to the combustion
chamber 20. Flow into the combustion chamber 20 is controlled by one or
more intake valves 21, operatively controlled by a valve actuation device

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comprising a lifter in conjunction with a camshaft (not shown). Combusted
(burned) gases flow from the combustion chamber 20 via the exhaust port 19.
with the flow of combusted gases through the exhaust port controlled by one
or more exhaust valves 23 operatively controlled by a valve actuation device
such as a second camshaft (not depicted). Specific details of a control scheme
to control opening and closing of the valves are not detailed. Valve actuation
and control devices, including hydraulic valve lifter devices, variable cam
phasers, variable or multi-step valve lift devices, and cylinder deactivation
devices and systems can be utilized to extend operating regions of the engine
and fall within the purview of the invention. Other generally known aspects of
engine and combustion control are known and not detailed herein. The engine
operation typically comprises conventional four stroke engine operation
wherein each piston reciprocates within the cylinder between top-dead center
(TDC) and bottom-dead center (BDC) locations defined by rotation of the
crankshaft 35. with opening and closing of the intake valves and exhaust
valves controlled during repetitive, sequential exhaust, intake, compression
and expansion strokes.
[0011] In one embodiment, the engine is an element of a hybrid powertrain
system comprising the engine, an electro-mechanical transmission, and a pair
of electric machines comprising motor/generators. The aforementioned
elements are controllable to selectively transmit torque therebetween, to
generate tractive or motive torque for transmission to a drivcline and to
generate electrical energy for transmission to one of the electrical machines or
to an electrical storage device.
[0012] The ECM is preferably an element of the overall control system 5
comprising a distributed control module architecture operative to provide
coordinated powertrain system control. The powertrain system control is
effective to control the engine to meet operator torque demands, including
power for propulsion and operation of various accessories. Communication
between the control system and the engine 10 is depicted generally as element

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45, and comprises a plurality of data signals and control signals that arc
transferred between elements of the engine and the control system. The ECM
collects and synthesizes inputs from sensing devices, including a MAP
(manifold absolute pressure) sensor 16, an engine crank sensor 31, an exhaust
gas sensor 40, and a mass airflow sensor (not shown), and executes control
schemes to operate various actuators, e.g., the fuel injector 12 and the ignition
module for spark ignition at the spark plug 14, to achieve control targets,
including such parameters as fuel economy, emissions, performance,
driveability, and protection of hardware. The ECM is preferably a general-
purpose digital computer generally comprising a microprocessor or central
processing unit, storage media comprising read only memory (ROM), random
access memory (RAM), electrically programmable read-only-memory
(EPROM), a high speed clock, analog-to-digital (A/D) and digital-to-analog
(D/A) conversion circuitry, and input/output circuitry and devices (I/O) and
appropriate signal conditioning and buffer circuitry. Control schemes,
comprising algorithms and calibrations, are stored as machine-executable code
in memory devices and selectively executed. Algorithms arc typically
executed during preset loop cycles such that each algorithm is executed at
least once each loop cycle. Algorithms stored as machine-executable code in
the memory devices are executed by the central processing unit and are
operable to monitor inputs from the sensing devices and execute control and
diagnostic routines to control operation of the respective device, using preset
calibrations. Loop cycles are typically executed at regular intervals, for
example each 3.125, 6.25, 12.5, 25 and 100 milliseconds during ongoing
engine and vehicle operation. Alternatively, algorithms may be executed in
response to occurrence of an event.
[0013] The invention comprises a simulation model that is stored as
machine-executable code and is regularly executed in the control system. The
simulation model is operative to calculate, in real-time, a cylinder pressure for
each cylinder as a function of engine crank angle. Cylinder pressure is

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generated by the action of crankshaft rotation wherein movements of the
pistons in the engine cylinders are resisted by air trapped within the
combustion chambers of the cylinders. Crank torque, i.e., torque exerted on
the crankshaft by each piston, is determined from the cylinder pressure. Total
engine crank torque is determined, comprising a sum of the cylinder torques
calculated for each cylinder. Each cylinder torque is determined by
multiplying a torque ratio by a cylinder pressure. The torque ratio is
determined for each cylinder as a function of crank angle, which encompasses
changes in cylinder geometry and cylinder friction. The torque ratio is
preferably a pre-calibrated array of values stored in memory, and retrievable
as based upon crank angle.
[0014] The simulation model generally comprises machine-executable code,
stored in the ECM or other control module of the control system, which
determines pressure in an unfired cylinder(s) of the internal combustion engine
during operation of the powertrain system when the engine is motoring, i.e.,
the engine crankshaft is rotating without spark ignition and fuel injection to
the cylinders. . The simulation model begins execution substantially
simultaneously with start of rotation of the stopped engine, or when engine
firing has stopped due to stoppage of engine fueling and/or spark ignition.
Such instances of operation occur when the engine is being started, or stopped,
or when specific cylinders are deactivated. Engine starting can comprise
rotation of the engine crankshaft for a period of time before introducing fuel or
spark ignition to cylinders. The pressure is preferably determined regularly
every few degrees of engine rotation, typically at least once every five degrees
of crankshaft rotation, or during each 6.25 ms loop cycle.
[0015] The code comprises determining an instantaneous measure of
combustion chamber volume, and determining positions of the intake and
exhaust valves. This includes determining cylinder pressure at each valve
transition. There are four valve transition events which occur during ongoing
engine operation, comprising intake valve opening (IVO). intake valve closing

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(IVC), exhaust valve opening (EVO) and exhaust valve closing (EVC).
Cylinder pressure for each unfired cylinder is determined based, upon the
combustion chamber volume, positions of the corresponding intake and
exhaust valves, and the cylinder pressure at a most recently occurring valve
transition.
[0016] The cylinder pressure is calculated, as described hercinbelow. The
general cylinder pressure equation is as follows in Eq. 1:
P2 = P1 * (V1 /V2)1.3 [1]
[0017] wherein P2 indicates cylinder pressure at the current timestep, and P1
indicates cylinder pressure determined at the most recently occurring valve
transition. Cylinder compression is approximated as an adiabatic
compression, i.e., having minimal or no heat transfer. The term V1 comprises
combustion chamber volume at the most recently previously occurring valve
transition, and V2 comprises the combustion chamber volume at the current
timestep, based upon a predetermined calibration comprising a range of
combustion chamber volumes determined based upon engine crank angle. An
algorithm operative to execute Eq. 1 is executed only when the intake and
exhaust valves are all closed, i.e., ValveState is ValvesClosed. Pressure and
torque calculations are preferably computed at the highest calculation rate, i.e.,
6.25 ms.
[0018] When the exhaust valves are open (i.e., ValveState is ExhaustOpen),
P2 is determined based upon a first-order lag filter leading to atmospheric
pressure. An overall assumption is that the airflow speeds are sufficiently low
that exhaust backpressure is at ambient atmospheric pressure. When the
intake valves arc open, P2 is determined based upon a first-order lag filter
leading to manifold pressure. An overall assumption of the model is that the
airflow speeds are sufficiently low enough that exhaust backpressure is fixed
at zero (0.0 kPa) for all calculations. When the valves are closed, necessary
data is calculated before the valves close. Eor forward engine rotation, the
intake valve is closing, P1 is initialized to manifold pressure (MAP) and V1 is

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calculated by using the angle for IVC and the calibration of combustion
chamber volume based upon engine crank angle. For reverse engine rotation.
the exhaust valve is closing. P1 is initialized to atmospheric pressure and V1 is
calculated by using the angle for EVO and the calibration of combustion
chamber volume based upon engine crank angle. A correction is also made
for leakage and blow-by past the piston, which is critical for low engine
speeds to achieve correct initial conditions, this comprises modifying the
value for P1 to P1adj to account for losses proportional to the pressure
difference between P1 and P2, this modification or adjustment comprising Hq.

wherein K is a calibratable system-specific filter coefficient or gain
factor.
[0019] The calibration of combustion chamber volumes (V1. V2) based
upon engine crank angle is preferably stored in RAM as a long indexed array
of the combustion chamber volume corresponding to engine crank angle to
enhance computational speed, allowing the control module executing the
simulation to determine the torque ratio from a precalibrated array index based
upon engine crank angle. The exponent function for (V1/ V2)1.3 is estimated
as a second-order polynomial for the ranges of representative volume ratios
(V1/V2 ranging from about 0.2 to 15), which provides a good practical fit and
dramatically reduces computational load. Key strategies to effect real-time
pressure and torque calculations include the previously described calibration
for combustion chamber volume based upon engine crank angle, and a
calibration for crank torque based upon cylinder pressure, which are
determined offline for the specific engine application and executed as
calibrations to minimize computational load.
[0020] Each opening and closing event of the intake and exhaust valves is
modeled as discrete, i.e., the valve is either open or closed. When one of the
valves is transitianed to open, the cylinder pressure is filtered to one of cither

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manifold pressure (MAP) or exhaust pressure. PEXHAUST- which is assumed to be
atmospheric pressure, as shown in Eq. 3:

wherein P2 indicates cylinder pressure at the current timestep, and P1
indicates cylinder pressure determined at the most recently occurring valve
transition. Each valve timing event requires accurate timing, preferably less
than five crank angle degrees of rotation. This includes speed-based
corrections which are made to account for airflow dynamics and pump-down
and leakage of valve lifters.
[0021] The effect of valve position and valve timing on cylinder pressure is
also modeled for inclusion in the control scheme. During ongoing engine
operation the four valve transition events, comprising intake valve opening
(IVO), intake valve closing (IVC), exhaust valve opening (EVO) and exhaust
valve closing (EVC), ongoingly occur. With regard to modeling cylinder
pressure, crank angle at which IVC occurs is critical, as this initiates engine
operation with all the valves closed when the engine is rotating in a positive
direction, and the combustion chamber is essentially a closed chamber with
pressure varying based upon volume of the combustion chamber. To limit
computational load, only factors significantly affecting IVC angle arc
modeled. Within the fastest computational loop (i.e., 3.125 ms) the simulation
model monitors crank angle for each cylinder and assigns a ValveStatc flag
which is set to one of IVO, EVO. and, Valves Closed (IVC and EVC). Valve
overlap is ignored because of the minor influence on crank torque. There are
two primary influences on IVC angle. Air flow dynamics arc a function of
engine speed and change the effective valve closing angle when modeling the
valve timing as 100% open or 100% closed.

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[0022] Furthermore, at low and zero engine speed, hydraulic valve lifters
tend to leak down on any valves that are in an open state, until either the valve
closes or the lifter fully collapses. As engine speed increases the velocity of
air exiting the valve increases. Therefore, the valve must open further for
similar pressure drop. This is addressed using computational flow dynamics
(CFD) simulations developed off-line executed with actual valve dynamics to
assess the maximum cylinder pressure achieved at piston top-dead-center
(TDC). The simplified model shown in Eq. 2 can be restated as Eq. 4:

wherein VIVc is combustion chamber volume at intake valve closing;
PTDC is cylinder pressure at top-dead-center;
PIVC is cylinder pressure at intake valve closing; and,
VTDC is combustion chamber volume at top-dead-center.
VIVC can be used to directly determine the crank angle at IVC, which
depicts valve lift at the equivalent IVC (EIVC) using a prccalibrated cam
profile calibration, IntakeProfile, to determine valve lift based upon crank
angle. An off-line simulation is preferably used to determine the calibration
table for valve lift based upon engine speed (IVCLift v RPM) at different
engine speeds. The data is curve-fit to determine a slope of lift at IVC, based
upon the engine speed. This calibration permits real-time determination of the
valve lift at which to transition the model from the intake valve being open
(IVO) to the intake valve being closed (IVC) by multiplying the calibration
value by the engine speed, as shown in Eq. 5:
EIVC Lift = RPM * IVCLift v RPM. [5]
[0023] Valve lifters can leakdown at slow engine speed and engine off,
which affects the effective valve timing at engine start. When a valve is open,
the valvetrain load is applied to the hydraulic lifter, which is not a perfectly
sealed device, resulting is fluid leaks and lifter and valve displacement. The
leakdown rate is highly variable with temperature, wear, and component
tolerances. The lifter leaks until it either bottoms out or the valve closes. The

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cylinder model typically does not track during the few seconds it takes the
lifter to leak down at zero speed, due to too many sources of variation.
However, control schemes typically transition cylinders to unfired operation
for longer than a few seconds, allowing the final position to be modeled
reasonably well.
[0024] In this embodiment, only the intake valve lifter is modeled to reduce
computational load and save time. The effect of exhaust valve timing on
compression torque is considered less critical. This is because opening of the
exhaust valve occurs at the end of the pressure estimation operation, and
closing of the exhaust valve is coincident with opening of the intake valve, and
outside of the pressure estimation window described with reference to Eq. 2,
above.
[0025] Based upon ValveState data, when the valve transition state
comprises IVO, or IntakeOpen, the lifter leakdown variable for that cylinder is
incremented. Data is typically provided in dimensions of millimeters (mm) of
lift and referenced to the cam profile. The leakdown variable is limited to a
calibrated value for maximum leakdown. When the ValveState changes to
ValvesClosed or ExhaustOpen then the lifter leakdown is reset to zero. For
the exhaust valve transitions, angles for EVO and EVC are fixed calibrations,
because variation in timing of either transition does not introduce enough final
torque error to warrant the calculations to model more completely. For the
intake valve transition, both IVO and IVC are adjusted. The IVO transition is
preferably calculated using a base calibration for IVO (BaseIVO) based upon
the cam profile diagram incremented by a factor based upon an approximate
slope of the cam opening (CamSlope) and the lifter leakdown
(LifterLeakdown):
IVO angle = BaselVO + CamSlope*LifterLeakdown

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[0026] The angle for IVC is calculated more accurately using both
LifterLeakdown and the lift required for effective IVC. The actual cam profile
is preferably used as a calibration to provide the intake valve profile,
IntakeProfile, based upon cam lift and camshaft angle. The total cam lift
where the intake valve is considered open is computed as:
Lift = EIVC_Lift + LifterLeakdown.
[0027] The angle for IVC can be looked up in the cam profile calibration,
IntakeProfile, at the calculated lift. This calculation typically occurs at one of
the slower loop cycle rates, with the data fed into the fast inner loop to
estimate cylinder pressures and assign valve state for each of the intake and
exhaust valves.
[0028] The calibration of torque ratio based upon crank angle,
TorqRatio Vs Angle, is preferably constructed offline and represents an
equivalent value for crank torque (in Nm) as a function of cylinder pressure
(in kPa) determined at each crank angle. The torque ratio parameters arc
developed for the specific engine design and configuration, and include factors
related to cylinder geometry and piston friction. A factor for torque ratio,
TorqRatio, can be determined from the calibration TorqRatio Vs Angle for
each cylinder as a function of crank angle. Thus, cylinder torque for a given
cylinder comprises the estimated cylinder pressure multiplied by the torque
ratio, i.e., CylTorq = TorqRatio * CylPres. Total crankshaft torque is
determined to be a sum of the cylinder torque values. CylTorq, for each of the
cylinders. The calibration of TorqRatioVs Angle is preferably stored in non-
volatile computer memory as an array to improve computational speed.
[0029] The real-time simulation model for determining cylinder compression
pressure preferably begins operating at or before the point in time at which the
engine crankshaft begins spinning, or after engine firing has been discontinued
precedent to stopping engine rotation. Thus by modeling valve timing,
generating calibration tables offline, and assuming simple adiabatic

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compression, the instantaneous torque applied to the crank can be accurately
estimated in real time in the control module.
[0030] Referring now to Fig. 2, a schematic block diagram of an overall
control scheme designed in accordance with an embodiment of the invention
is provided. The control scheme described is preferably executed using an
embedded controller in the control system described herein. The control
system preferably executes the control scheme when there is a need for
information related to cylinder pressure including engine crank torque, for
purposes of engine or powertrain control, such as during starting of the engine,
or during engine shutdown. The control scheme may also be executed when
one or more of the cylinders are deactivated.
[0031] There are two functional elements of the overall control scheme,
comprising a control scheme operative to calculate cylinder torque and
pressure, depicted as CalCylTorqPress, and a control scheme operative to
calculate cylinder data, depicted as CalcCylData.
[0032] The CalcCylData control scheme is preferably executed each 25 ms
loop cycle for each engine cylinder when enabled, such as during an engine-
start operation. Inputs to the CalcCylData control scheme comprise the
number of engine cylinders (NumCyls), crankcase pressure (CrankCasePress).
engine intake manifold pressure (MAP), engine speed (EngkPM), exhaust
system pressure (ExhaustSysPress). Further inputs include the lifter state
(LifterState) and current cylinder pressure (CylPres) for the selected engine
cylinder, which are outputs from the CalCylTorqPress control scheme.
Another input comprises the precalibrated array of combustion chamber
volume determined as a function of engine crank angle (DispVsAngle). From
the inputs previously described, various outputs of the CalcCylData control
scheme are determined and input to CalCylTorqPress control scheme. The
outputs comprise intake valve opening angle (Phi_IntVlvOpen). intake valve
closing angle (Phi_IntVlvCls), an initial combustion chamber volume
(InitialCylVol), and an initial cylinder pressure (InitialCylPrs) for the cylinder.

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[0033] The CalCylTorqPress control scheme is preferably executed during
each 6.25 ras loop cycle for each engine cylinder when enabled. Inputs to the
CalCylTorqPress control scheme comprise states of parameters typically based
upon measurements, including engine crank angle (CrankAngle), and engine
intake manifold pressure (MAP). Other engine states that are determined
comprise crank case pressure (CrankCasePress) and exhaust system pressure
(ExhaustSysPress). Further values include exhaust valve opening angle
(Phi ExhVlvOpen) comprising a predetermined calibration for torque ratio
determined based upon crank angle (TorqRatioVsAngle), a predetermined
calibration for combustion chamber displacement based upon crank angle
(DispVsAngle), and the number of cylinders (NumCyls). Furthermore, the
inputs from CalcCylData control scheme, including intake valve opening
angle (Phi IntVlvOpen), intake valve closing angle (Phi IntVlvCls), an initial
combustion chamber volume (InitialCylVol), and an initial cylinder pressure
(InitialCylPrs) are provided.
[0034] The CalCylTorqPress control scheme is configured to manipulate the
inputs described to calculate and determine the outputs, including the cylinder
pressure and crankshaft torque (TotalCrankTorq) using the equations and
calibrations described hereinabove during ongoing operation, when the control
scheme is enabled to do so.
[0035] Alternate embodiments are allowable within the scope of the
invention, including systems employing valve management devices such as
variable cam phasing. In an embodiment employing variable cam phasing, the
cam phasing is preferably locked into a park position during execution of the
simulation model. The park position can be either a full cam advance
position, or a full cam retard position, preferably the full cam retard position to
minimize magnitude of compression pulses.

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[0036] The specific details of the control schemes and associated results
described herein are illustrative of the invention as described in the claims.
The invention has been described with specific reference to the embodiments
and modifications thereto. Further modifications and alterations may occur to
others upon reading and understanding the specification. It is intended to
include all such modifications and alterations insofar as they come within the
scope of the invention.

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Having thus described the invention, it is claimed:
1. Article of manufacture, comprising a storage medium having a machine-
executable program encoded therein to determine pressure in an unfired
cylinder of an internal combustion engine the cylinder comprising a variable
volume combustion chamber defined by a piston reciprocating within the
cylinder between a top-dead center position and a bottom-dead center position
and an intake valve and an exhaust valve controlled during repetitive,
sequential exhaust, intake, compression and expansion strokes, said piston
operatively connected to a rotatable engine crankshaft, the program
comprising:
code to determine volume of the combustion chamber;
code to determine positions of the intake and exhaust valves;
code to determine a parametric value for cylinder pressure at each valve
transition; and,
code to estimate cylinder pressure based upon the combustion chamber
volume, positions of the intake and exhaust valves, and the cylinder
pressure at a most recently occurring valve transition.
2. The article of claim 1, wherein the code to determine the volume of the
combustion chamber comprises code to select combustion chamber volume
from a precalibrated array of combustion chamber volumes indexed to a
rotational position of the engine crankshaft.
3. The article of claim 1, wherein the code to determine a parametric value
for cylinder pressure at each valve transition comprises code to estimate the
cylinder pressure based upon intake manifold pressure subsequent to opening
the intake valve.

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4. The article of claim 1, wherein the code to determine a parametric value
for cylinder pressure at each valve transition comprises code to estimate the
cylinder pressure based upon atmospheric pressure subsequent to opening the
exhaust valve.
5. The article of claim 1, wherein the code to estimate the cylinder pressure
based upon combustion chamber volume, valve position, and the cylinder
pressure at each valve transition comprises code to estimate the cylinder
pressure based upon atmospheric pressure when the exhaust valve is open.
6. The article of claim 1, wherein the code to estimate the cylinder pressure
based upon combustion chamber volume, valve position, and the cylinder
pressure at each valve transition comprises code to estimate the cylinder
pressure based upon manifold pressure subsequent to opening the intake valve.
7. The article of claim 1, wherein the code to estimate the cylinder pressure
based upon combustion chamber volume, valve position, and the cylinder
pressure at each valve transition comprises code to determine the cylinder
pressure based upon a cylinder compression ratio subsequent to closing the
intake valve.
8. The article of claim 7, further comprising:
code to determine the cylinder compression ratio based upon an adiabatic
approximation of a volumetric ratio between the current combustion
chamber volume and the combustion chamber volume at the most
5 recently previously occurring valve transition; and,
code to determine the current cylinder pressure based upon the cylinder
compression ratio.

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9. The article of claim 1, wherein the code is executed to determine
pressure in the unfired cylinder during engine motoring prior to firing the
engine.
10. The article of claim 9, wherein execution of the machine-executable
code begins substantially simultaneously with beginning of rotation of the
engine.
11. The article of claim 10, further comprising repetitively executing the
machine-executable code at least once every five degrees of crank angle
rotation prior to firing the engine.
12. The article of claim 1. wherein the code is executed to determine
pressure in the unfired cylinder during engine motoring after discontinuing
firing the engine.
13. The article of claim 1, further comprising code to adjust the estimated
cylinder pressure based upon engine rotational speed.
14. The article of claim 1, further comprising code to adjust the estimated
cylinder pressure based upon leakdown of the intake valve.
15. Article of manufacture, comprising a storage medium having a machine-
executable program encoded therein to determine engine crank torque in an
unfired multi-cylinder internal combustion engine comprising a plurality of
variable volume combustion chambers each defined by a piston reciprocating
5 within one of the cylinders between top-dead center and bottom-dead center
positions and an intake valve and an exhaust valve controlled during
repetitive, sequential exhaust, intake, compression and expansion strokes, each

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piston operatively connected to a rotatable engine crankshaft, the program
comprising:
code to determine volume of each of the combustion chambers;
code to determine positions of the intake and exhaust valves;
code to determine a cylinder pressure at each valve transition;
code to estimate cylinder pressure for each cylinder based upon the
combustion chamber volume, positions of the intake and exhaust
valves, and the cylinder pressure at a most recently occurring valve
transition;
code to determine a cylinder crank torque for each cylinder based upon the
estimated cylinder pressures; and,
code to determine an overall crank torque based upon the cylinder crank
torques for each of the cylinders.
16. The article of claim 15, wherein the code to determine engine
compression torque during the engine rotation comprises an engine
compression torque simulation executed as one or more computer programs in
the article of manufacture.
17. The article of claim 16, further comprising the engine compression
torque simulation to predict engine torque over a range of ambient and engine
operating conditions.
18. The article of claim 15, wherein the code to estimate cylinder pressure
based upon combustion chamber volume, valve position, and the cylinder
pressure at each valve transition comprises code to determine the cylinder
pressure based upon a cylinder compression ratio subsequent to closing the
intake valve.

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19. The article of claim 18, further comprising:
code to determine the cylinder compression ratio based upon an adiabatic
approximation of a volumetric ratio between the current combustion
chamber volume and the combustion chamber volume at the most
recently previously occurring valve transition; and,
code to determine the current cylinder pressure based upon the cylinder
compression ratio.
20. Method to determine pressure in an unfircd cylinder of an internal
combustion engine the cylinder comprising a variable volume combustion
chamber defined by a piston reciprocating within a cylinder between top-dead
center and bottom-dead center positions and an intake valve and an exhaust
valve controlled during repetitive, sequential exhaust, intake, compression and
expansion strokes, said piston operatively connected to a rotatable engine
crankshaft, the method comprising:
determining volume of the combustion chamber;
determining positions of the intake and exhaust valves;
determining cylinder pressure at each valve transition; and,
estimating cylinder pressure based upon the combustion chamber volume.
positions of the intake and exhaust valves, and the cylinder pressure at
a most recently occurring valve transition.
21. The method of claim 20. wherein estimating cylinder pressure based
upon cylinder volume, valve position, and the cylinder pressure at each valve
transition comprises determining the cylinder pressure based upon a cylinder
compression ratio subsequent to closing the intake valve.

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22. The method of claim 21, further comprising determining the cylinder
compression ratio based upon an adiabatie approximation of a volumetric ratio
between the current combustion chamber volume and the combustion chamber
volume at the most recently previously occurring valve transition; and,
determining the current cylinder pressure based upon the cylinder compression
ratio.

An article of manufacture and method are provided to determine pressure in an unfired cylinder of an internal combustion engine. The cylinder comprises a variable volume combustion chamber defined by a piston reciprocating within a cylinder between top-dead center and bottom-dead center points and an intake valve and an exhaust valve controlled during repetitive, sequential exhaust, intake, compression and expansion strokes of said piston. The code is executed to determine volume of the combustion
chamber, and determine positions of the intake and exhaust valves. A parametric value for cylinder pressure is determined at each valve transition. Cylinder pressure is estimated based upon the combustion chamber volume, positions of the intake and exhaust valves, and the cylinder pressure at the most recently occurring valve transition.